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Science: Prehistoric farming caused ‘devastating’ soil erosion

By SARAH BUNNEY

THE EROSION of tropical soils on hillsides where trees have been cut
down is a common sight today. Now, scientists are finding that people have
been causing such damage for several thousand years, and the affected land
has never recovered.

In the highlands of central Mexico there have been several episodes
of devastating soil erosion since people began to cultivate maize on a wide
scale 3500 years ago. At times, soil erosion has been so severe that it
has forced people to abandon their settlements.

Evidence for such prehistoric degradation comes primarily from lake
sediments. Alayne Street-Perrott and Alan Perrott, of the University of
Oxford, and Douglas Harkness of the Natural Environmental Research Council’s
Radiocarbon Laboratory in Glasgow, have been studying the sediment around
Lake Patzcuaro to the west of Mexico City. They have discovered two devastating
episodes of soil erosion in prehistoric times. One began about 2300 years
ago and the other around 1000 years ago (American Antiquity, vol 54, p 759).

The effects of this destruction can be seen today on the steep-gullied
hillsides that surround the lake and in the fans of red soil redeposited
around the lake shore. Where once there was abundant pine forest, now there
is only impoverished shrub. Only small pockets of forest have regenerated.
The local fisheries are also threatened because the lake is becoming eutrophic
– enriched with nutrients from the soil washed into the lake.

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The two major phases of soil erosion were preceded by a minor event
about 3500 years ago, in the early days of maize cultivation. Sarah O’Hara
at the University of Oxford has found signs of this erosion ‘blip’ in cores
of lake sediment. Although people cleared only a limited area of trees,
the soil was washed into the lake, starting the process that would make
it eutrophic.

The first serious ecological disaster to hit the Patzcuaro Basin began
about 2300 years ago. By this time, say Street-Perrott and her colleagues,
people were living in villages, and had built small ceremonial centres.
Farmers cultivated only areas of the lake shore and the lower slopes. As
a result, there was considerable overcultivation.

When the farmers cleared and burnt the pine forests on the steep hillsides
around the lake, they caused so much devastation that huge amounts of red
topsoil were washed down into the lake. In exposed sediments on the lakeside,
Street-Perrott and her colleagues found a layer of eroded forest soil. The
layer is rich in tiny fragments of pine charcoal from the burning. So severe
was the erosion at the northern end of the lake basin, say the researchers,
that the area became completely depopulated.

A second, more intense phase of erosion in the Lake Patzcuaro area also
shows up in the lake sediments. This probably began between 900 and 1000
years ago, with the arrival of the Purepecha, a warlike people who were
similar to the Aztecs.

This period of extensive erosion continued into the late 16th and 17th
centuries. People burnt the forest for a variety of reasons.

Increasingly, there is evidence that the cycles of soil erosion around
the lake were repeated around other centres of population in the region.
Street-Perrott says: ‘Soil erosion has to be taken very seriously as a cause
of the collapse of city states and the shifting of power from centre to
centre in Mesoamerica in pre-Hispanic times.’